Bulk flow of halos in \Lambda CDM simulation

TL;DR

This paper validates that halo bulk flows in DM simulations follow Maxwellian distributions and compares these predictions with observations, revealing moderate deviations and insights into the relationship between bulk flow, mass distribution, and halo clustering.

Contribution

It demonstrates the Maxwellian nature of halo bulk flows in DM simulations and proposes a method to compare observed bulk flows with theoretical predictions at an effective scale.

Findings

Bulk flow variance matches linear theory predictions.

Some observed bulk flows deviate from DM expectations at ~3 level.

Bulk flow shows weak correlation with internal mass dipole and little dependence on halo mass.

Abstract

Analysis of the Pangu N-body simulation validates that the bulk flow of halos follows a Maxwellian distribution which variance is consistent with the prediction of the linear theory of structure formation. We propose that the consistency between the observed bulk velocity and theories should be examined at the effective scale of the radius of a spherical top-hat window function yielding the same smoothed velocity variance in linear theory as the sample window function does. We compared some recently estimated bulk flows from observational samples with the prediction of the \Lambda CDM model we used; some results deviate from expectation at a level of ~ 3\sigma but the discrepancy is not as severe as previously claimed. We show that bulk flow is only weakly correlated with the dipole of the internal mass distribution, the alignment angle between the mass dipole and the bulk flow has a broad distribution peaked at ~ 30-50 deg., and also that the bulk flow shows little dependence on the mass of the halos used in the estimation. In a simulation of box size 1Gpc/h, for a cell of radius 100 Mpc/h the maximal bulk velocity is >500 km/s, dipoles of the environmental mass outside the cell are not tightly aligned with the bulk flow, but are rather located randomly around it with separation angles ~ 20-40 deg. In the fastest cell there is a slightly smaller number of low-mass halos; however halos inside are clustered more strongly at scales > ~ 20 Mpc/h, which might be a significant feature since the correlation between bulk flow and halo clustering actually increases in significance beyond such scales.